The present invention relates in general to manufacturing a monomode preform and to the corresponding monomode optical fiber. It is applicable to the field of optical fiber telecommunications. By way of non-limiting example, it applies to polymer, fluoride glass, etc . . . technologies. In particular, the invention can be used in making economically viable optical amplifiers possessing low splice losses between the amplifying fiber portion and the line fibers, and possessing spectral attenuation compatible with the required pumping efficiencies.
Optical fiber manufacture comprises two successive steps, namely making a preform, and drawing said preform down to obtain an optical fiber.
Chemical vapor deposition (CVD) type techniques as summarized in the document entitled xe2x80x9cPreform technologies for optical fibersxe2x80x9d by D. Dorn and C. Le Sergent, published in xe2x80x9cElectrical Communicationxe2x80x9d, Vol. 62, No. 3/4, 1988, can be used for making a preform out of polymer or fluoride glass, but that does not give satisfactory results, particularly in terms of attenuation. Under such circumstances, a technique of the xe2x80x9cbuilt-in castingxe2x80x9d type is then used, which technique is well known to the person skilled in the art and is described in the article by S. Mitachi, T. Miyashita, and T. Kanamori published in Electronics Letters, Vol. 17, pp. 591 et seq. (1981). That technique uses a hollow cylindrical mold having a closed bottom. Molten glass material for forming the cladding is poured into the mold. Before the cladding glass material forming a solid cylinder in the mold has solidified completely, a cylindrical central portion is removed from the solid cylinder and the resulting void is filled in with molten core glass material. As a result, after cooling, an initial preform is obtained in the form of a bar having a central core zone and a peripheral cladding zone. During the fiber-drawing step, the optical fiber is drawn from said preform. The fiber comprises both a core and cladding surrounding the core. The ratio of core diameter to fiber diameter is identical to the ratio of core zone diameter to preform diameter. Diameter ratio (core diameter/total diameter) is thus conserved between preform and fiber.
To obtain a monomode optical fiber, i.e. one in which only one mode propagates, it is necessary for the fiber to possess a core of diameter that is small in terms of the wavelengths used. Unfortunately, the xe2x80x9cbuilt-in castingxe2x80x9d technique, like the so-called xe2x80x9csuction castingxe2x80x9d and xe2x80x9crotational castingxe2x80x9d techniques respectively described in articles by Y. Ohishi, S. Sakaguichi, and S. Takahashi, Electronics Letters, Vol. 22, pp. 1034 et seq. (1986) and by D. C. Tran, C. F. Fisher, and G. H. Sigel, Electronics Letters, Vol. 18, pp. 657 et seq. (1982), does not enable an initial preform to be made having a core diameter that is sufficiently small. In a known method, the initial preform is stretched to form a xe2x80x9cmotherxe2x80x9d preform possessing a core zone of diameter smaller than that of the initial preform. This mother preform is then inserted into an outer tubular sleeve of cladding glass, such that after drawing down, an optical fiber is obtained possessing the desired ratio of core diameter to fiber diameter. For example, the outer tube may be obtained by centrifuging or xe2x80x9crotational castingxe2x80x9d. If the outer tube does not enable the desired ratio to be obtained, a second mother preform can be made by stretching the first mother preform with its outer tube. This second mother preform is then inserted into a second outer tube. The operation of inserting a mother preform into an outer tube can thus be repeated as many times as necessary to achieve the desired ratio. It is also possible to insert the mother preform into a plurality of concentric outer tubes and to draw the fiber from the resulting assembly.
The fiber-drawing step consists in heating one of the ends of the assembly made up of mother preform and outer tube and in drawing an optical fiber from the preform that is melted in this way by heating. As shown in FIG. 1, which is a longitudinal section through such an assembly 1, the mother preform 11 is inserted into the outer tube 10 and leaves an empty volume 12 between the cylindrical outside surface of the mother preform 11 and the cylindrical inside surface of the outer tube 10. During the fiber-drawing step, very intense heating is applied to the assembly 10-11 so as to obtain local melting in the peripheral cladding zone 110 of the mother preform 11 and in the outer tube 10 so as to fill in the initially empty volume 12.
This intense heating suffers from the major drawback of imparting deformation and eccentricity to the core zone 111 of the mother preform 11, and consequently of imparting deformation and core eccentricity to the resulting optical fiber, and this is very damaging to the quality of the fiber. For example, such deformation and core eccentricity of the fiber leads to high losses when making splices between a line fiber of silica glass and a portion of amplifying fiber of fluoride glass in order to make an optical amplifier.
The present invention seeks to remedy the above-mentioned drawback. A first object of the invention is to provide a monomode preform suitable for making a monomode optical fiber. A second object of the invention is to provide a method of manufacturing such a monomode optical fiber. Another object of the invention is to provide a monomode optical fiber possessing a core that is substantially circular and centered.
To this end, according to the invention, a monomode preform comprising a mother preform housed in an outer tube and leaving an empty volume between an outside surface of the mother preform and an inside surface of the outer tube, is characterized in that an intermediate tube is inserted in said empty volume, the intermediate tube possessing viscosity at fiber-drawing temperature which is less than the viscosity(ies) at fiber-drawing temperature of said mother preform and of said outer tube.
In an embodiment, the preform is made of fluoride glass, said intermediate tube possessing a vitreous transition temperature lower than the vitreous transition temperature(s) of said mother preform and of said outer tube.
The core zone of the mother preform may be doped with rare earth ions.
The invention also provides a method of manufacturing a monomode optical fiber comprising the step of drawing a fiber from a monomode preform of the above-described type. By way of example, the fiber-drawing temperature lies substantially in the range 320xc2x0 C. to 330xc2x0 C.
To manufacture a doped monomode optical fiber, fiber-drawing is performed on a preform whose core zone is doped.
Such a doped optical fiber can be used to make an optical amplifier or a laser source.